FIG. 1 shows an example of conventional optical fiber light-amplifier system. A conventional single-mode optical fiber 21 through which input signal light is propagated is coupled to a first input 61 of an optical coupler 2, and a similar single-mode optical fiber 22 for propagating pumping laser light is coupled to a second input 62 of the optical coupler 2. For example, an erbium-doped optical fiber 4, which provides an optical fiber for light amplification, is coupled to the output 63 of the optical coupler 2. The output end of the erbium-doped optical fiber 4 is coupled to an ordinary single-mode output optical fiber 23 for transmission of amplified signal light.
The optical fibers 21, 22 and 23 comprise cores 31, 32 and 33 having circular cross-sections, and claddings 41, 42 and 43 having annular cross-sections surrounding the associated cores 31, 32 and 33, respectively. In general, the cores and claddings are formed of quartz glasses or the like having different indexes of refraction. The optical fibers are protected by respective protecting coatings covering the claddings.
Similarly, the light amplifying optical fiber 4 comprises a core 34 having a circular cross-section and a cladding 44 having an annular cross-section, and the core 34 and the cladding 44 are formed of quartz glasses or the like having different indexes of refraction. The core 34 is doped with ions of a rare earth element, such as Er (erbium), Nd (neodymium), Yb (ytterbium), Sm (samarium), and Pr (praseodymium). The rare earth element to be used for the doping is determined in accordance with the wavelength of the input signal light and the wavelength of the pumping laser light. For example, for input signal light having a wavelength of 1.55 .mu.m and pumping laser light having a wavelength of 1.48 .mu.m, the core 34 is doped with Er ions. The optical fiber 4 is protected by a protecting coating covering the cladding 44, as in the case of the optical fibers 21, 22 and 23.
In operation, input signal light 6 having a wavelength of 1.55 .mu.m transmitted through the conventional single-mode optical fiber 21 from a remotely located semiconductor laser device (not shown) and enters into the optical coupler 2. Pumping laser light 10 having a wavelength of 1.48 .mu.m and emitted by a pumping, high output semiconductor laser device 8 is focused by a condenser lens 12 and enters into the core 32 through the end of the pumping laser light transmitting optical fiber 22. The 1.48 .mu.m wavelength pumping light 14 applied to the optical fiber 22 is propagated therethrough and applied to the optical coupler 2.
Within the optical coupler 2, the 1.48 .mu.m wavelength pumping light 14 and the 1.55 .mu.m wavelength input signal light 6 are introduced into the same waveguide and, thereafter, outputted to the Er-doped optical fiber 4. As is well-known, by pumping the core 34 of the Er-doped optical fiber 4 with pumping light 14 at a wavelength of 1.48 .mu.m, input signal light at a wavelength of 1.55 .mu.m is amplified through stimulated emission, and amplified input signal light 16 at a wavelength of 1.55 .mu.m is fed to the conventional single-mode output optical fiber 23 through which it is propagated to another location. If necessary, a filter (not shown) which allows only light of a desired wavelength to pass therethrough may be disposed before the optical fiber 23.
In the conventional optical fiber light-amplifier system with the above-described arrangement, in order to obtain the desired amplification, it is necessary to couple high output power pumping laser light from the high output semiconductor laser device 8 to one end of the core 32 of the single-mode optical fiber 22. The diameter of the core 32 of the optical fiber 22 is only several microns, for example, from about 5 .mu.m to about 10 .mu.m at most. Accordingly, only high output semiconductor laser light having its transverse modes sufficiently controlled in a high power output operating state of the laser device can efficiently enter as pumping light into the core 32. However, in the state of art, it is very difficult to provide a laser device which can produce pumping laser light with high output power and stable transverse modes.
An object of the present invention is to provide an optical fiber light-amplifier system in which efficient pumping can be provided with a high output power semiconductor pumping laser device which provides laser light without a high degree of control of its transverse modes.